Microarray and micro-bead technologies can be used as tools to conduct biological, chemical or biochemical analyses in a parallel, massively parallel or multiplexed fashion because of the large number of different compounds or substances that can be fabricated or deposited on the microarray substrate or beads. As is also well known in the art, microarrays and micro-bead technologies are applicable to a variety of such analyses including, but not limited to, mRNA or protein expression profiling, parallel DNA sequencing, protein-protein interaction mapping, protein-drug interaction analysis, antibody specificity testing, enzyme substrate profiling and single nucleotide polymorphism (SNP) detection as well as various other applications in the fields of biomarker discovery, diagnostics, prognostics, personalized medicine, protein interaction analysis, drug discovery and proteomics (See for example [Ramachandran et al. (2004) Science 305, 86-90; Zhu et al. (2001) Science 293, 2101-2105; MacBeath & Schreiber. (2000) Science 289, 1760-1763; Zhu et al. (2000) Nat Genet. 26, 283-289; Michaud et al. (2003) Nat Biotechnol 21, 1509-1512; Sheridan. (2005) Nat Biotechnol 23, 3-4; Robinson et al. (2003) Nat Biotechnol 21, 1033-1039; Robinson et al. (2002) Nat Med 8, 295-301; Xiao et al. (2007) Bioimformatics 23, 1459-1467; Hughes et al. (2007) Anticancer Res 27, 1353-1359]).
The plurality of compounds or substances arrayed or displayed on the microarray substrate or micro-beads can be of a variety of types and for a variety of uses. These compounds or substances are not intended to be limited to any one type or for any one use, and henceforth will be referred to “features”, as is commonly used in the art of microarrays. Microarray or micro-bead features can include, but are not limited to proteins, peptides, DNA, nucleic acids, nucleosides, nucleotides or polymers thereof, drug or other chemical compounds, polymers, cells, tissues, particles, nanoparticles or nanocrystals. Microarray or micro-bead features may be used as, for example, analytes, probes or targets in various applications, assays or analyses.
Microarrays currently exist as two-dimensional feature arrays fabricated on solid glass (plain or chemically activated/modified) or nylon substrates for instance. A variety of additional substrates such as nitrocellulose, polystyrene, polymeric or metallic materials provided as solid substrates, coatings, films, membranes or matrices are also available. Due to the massively parallel or multiplexed nature of microarrays, far more information is obtained from a single experiment compared to other non-parallel or non-multiplexed methods. Furthermore, because the samples to be analyzed are generally in limited supply, hard to produce and/or expensive, it is highly desirable to perform experiments on as many components in a mixture as possible on as many features as possible, on a single microarray. This calls for a significant increase in feature density and quantity on a single substrate. In general, microarrays with densities larger than 400 features per square centimeter are referred as “high density” microarrays, otherwise, they are “low density” microarrays. Affymetrix Inc. (Santa Clara, Calif.) for example, currently offers several commercial high density oligonucleotide microarrays having as much as 1 million or more ˜10 μm features, for feature densities reaching ˜1 million/cm2 [Barone et al. (2001) Nucleosides Nucleotides Nucleic Acids 20, 525-531]. Applications of these commercial microarrays include mRNA expression profiling or single nucleotide polymorphism (SNP) detection.
Production of microarray or micro-bead features can be achieved by a variety of methods, either by in situ production, or by deposition/binding of off-line produced feature substances onto microarray substrates, beads or particles. Current methods however, suffer from various deficiencies.
For microarrays, there are two categories of techniques on the market, photolithographic and mechanical printing. Photolithography is an in situ method, while mechanical printing techniques require off-line production of the feature substances followed by deposition of the features onto the microarray substrate. The photolithographic technique adapts the same fabrication process used for electronic integrated circuits, in order to in situ synthesize compounds or substances, monomer-by-monomer for example (e.g. nucleic acid monomers), directly on the microarray substrate. This technique requires a large capital outlay for equipment, running up to hundreds of millions of dollars. The initial setup of new microarray designs is also very expensive due to the high cost of producing photo masks. This technique is therefore only viable in mass production of standard microarrays at a very high volume. Even at high volumes, the complexity in synthesis still limits the production throughput resulting in a high microarray cost. This method has typically been employed for high density DNA microarrays. The complexity of the process however, also limits the length of the synthesized DNA to the level of short oligonucleotide sequences of about 25 bases.
The established mechanical printing technique [U.S. Pat. No. 5,807,522] uses a specially designed mechanical robot, which produces a feature spot on the microarray by dipping a pin head into a fluid, i.e. the bulk stocks of the feature substances, such as DNA or protein solutions, and then printing it onto the substrate at a predetermined position. Washing and drying of the pins are required prior to printing a different feature onto the microarray substrate. In current designs of such robotic systems, the printing pin, and/or the stage carrying the microarray substrates move along the XYZ axes in coordination to deposit samples at controlled positions on the substrates. Other mechanical printing techniques, either contact or non-contact, use quills, pins with built-in sample channels, non-contact ink jet/piezoelectric devices or capillaries as the means of feature deposition. Because a microarray contains a very large number of different features, these techniques, although highly flexible, are inherently very slow. Even though the speed can be enhanced by employing multiple pin-heads (or printing devices) and printing multiple substrates before washing, production throughput remains very low. Furthermore, the printing instrumentation is susceptible to mechanical failure due to the large number of moving parts. Non-contact methods additionally suffer from difficulties in controlling the microarray quality. Mechanical printing methods are therefore not suitable for high volume mass production of microarrays.
Mechanical printing also requires that the materials comprising the features be produced off-line, prior to printing. Typically, bulk stocks of the feature substances are produced and used to print multiple spots and/or microarrays. However, such production has a variety of limitations. For example, conventional off-line production of DNA (e.g. oligonucleotides) uses chemical synthesis, but is limited to approximately 150 bases in length, and although can be done in parallel, is not truly multiplexed. Conventional methods for DNA production beyond this length (e.g. full-length genes or large portions thereof), involves slow, laborious, and non-multiplexed standard DNA cloning practices. Adams and Kron [U.S. Pat. No. 5,641,658] disclose a general multiplexed method for producing DNA on beads or other surfaces by using solid-phase bridge PCR (i.e. where both PCR primers, forward and reverse, are attached to the surface). However, this approach is rarely used and has not been adapted for cloning (amplification of single template molecules) or downstream production of protein, for example. For recombinant proteins for instance, off-line production typically involves all the aforementioned conventional DNA cloning procedures in addition to labor intensive and non-multiplexed steps such as transfection, cell culture and purification reactions for each protein species. It is particularly important yet challenging to deposit the produced feature substances in pure and active form on the microarray substrate. Prior to deposition, feature substances are usually produced in heterogeneous mixtures and hence require purification. The production, purification and deposition process can readily inactivate delicate feature substances such as proteins. Furthermore, contaminants on the microarray surface can yield false signals in downstream analyses.
Feature size is another limiting factor of high density microarray production. With either microarray fabrication technique, photolithography or mechanical printing, the microarrays cannot easily be extended to spot sizes (i.e. features) at the nanometer level. Such nanoarrays would be highly advantageous, since they could dramatically increase the level of multiplexing for example. Photolithography represents the state-of-the-art in terms of spot size (10 μm) and density, but is limited to short polymers such as oligonucleotides and short peptides, and is essentially only used in practice for DNA microarrays.
Micro-bead technologies are analogous to microarrays except that the features are spatially segregated on different beads or particles. The experiment, analysis and/or readout can be formatted like a microarray, for example, with the beads arrayed or embedded on the surface or in wells of a device such as a microscope slide or plate. The experiment, analysis and/or readout can alternatively be performed with the beads suspended in a solution for example. The working density of features for micro-bead technologies is potentially far greater than for microarrays, depending primarily on the minimum usable bead size and maximum usable bead concentration or density. For example, 0.3 μm beads have been arrayed in etched wells at densities of 4×109 beads/cm2 [Michael et al. (1998) Anal Chem 70, 1242-1248], three orders of magnitude better than the current high density DNA microarrays from Affymetrix Inc. (Santa Clara, Calif.). However, because the beads are random, a decoding method is typically required to determine the identity of the feature on each bead in a given experiment or analysis. Several commercial entities utilize micro-bead technologies to achieve parallel or multiplexed assays in a fashion similar to microarrays. For example, Luminex Corporation (Austin, Tex.) markets a flow cytometry based bead platform for multiplexed assays, such as SNP detection and various immunoassays. Beads are fluorescently coded to facilitate the multiplexing and production of the bead “features”, e.g. analytes, is up to the end-user. Illumina Incorporated (San Diego, Calif.) has created a bar-coded bead-array platform for genetic analyses, such as multiplexed SNP and DNA methylation detection. 454 Life Sciences™ (Branford, Conn.) offers a bead-based parallel sequencing platform whereby beads carrying the DNA “features”, in this case DNA analytes for sequencing, are arrayed in microscopic wells and analyzed by massively parallel DNA pyrosequencing, for applications such as whole genome sequencing and detection of low abundance mutations.
In general, production of a plurality of beads with different features, whether the features are to serve as probes, targets or analytes for example, suffers from analogous problems as described for microarrays. For instance, different feature substances are typically produced off-line and can then be bound to beads in separate reactors, in a mechanical process of mixing solutions containing the feature substances with beads containing some binding capacity. This can be done in separate test tubes, vials or wells of a microtiter plate for example. Liquid handling robotics may be used to perform this process in parallel, however, it is again not truly multiplexed (e.g. does not produce the complete population of beads with different features, using a single reaction or few reactions within a single reactor).
The present invention overcomes the problems and disadvantages associated with current strategies and designs for the fabrication and utilization of microarrays, micro-bead technologies and a variety of other parallel, massively parallel or multiplexed biological sensing methodologies or devices.